Cycling Behavior of Iron Electrodes in Alkaline Batteries

Iron based alkaline battery systems such as nickel-iron and iron-air batteries are very attractive candidates for grid-scale energy storage applications [1]. Iron, which is the primary raw material for these batteries, is globally abundant, extremely inexpensive and environmentally friendly. The three key technical requirements for grid-scale energy storage systems are: high round-trip energy efficiency, good discharge rate capability and long cycle life [1, 2]. In a recent communication, we have reported a high-performance iron electrode that operates at a high charging efficiency of 92% and is capable of sustaining up to 3C rates during discharge [3]. Batteries for large scale energy storage applications are expected to last for at least 5000 cycles of charge and discharge. Nickel – iron batteries have been shown to withstand up to 2000 cycles of repeated cycling without significant degradation [4]. However, these batteries tend to be limited by the positive electrode and therefore an understanding of the cycling stability of the iron electrode is currently not available. The goal of the present study is to understand the robustness of the iron electrode and its stability when repeatedly subjected to charge and discharge. The iron electrodes used in this study were prepared by mixing carbonyl iron powder with polyethylene binder and hot-pressing the mixture onto a nickel grid. Electrochemical evaluation of the iron electrodes were performed in a three-electrode set up with nickel oxide counter electrodes and a mercury/mercuric oxide reference electrode [3]. Cycling Studies: A fully formed iron electrode had a discharge capacity of about 0.3 Ah/g and a charging efficiency of more than 95%. The change in discharge capacity of the iron electrode with cycling is shown in Figure 1. During the first 40 cycles, the discharge capacity of the iron electrode was found to be stable. However, upon prolonged cycling, a gradual fade in the discharge capacity was observed. The discharge capacity had decreased by up to 50% after 150 cycles of charge and discharge. It was also observed that the capacity of the iron electrode at high discharge rates had also decreased significantly with cycling (Figure 2). After 95 cycles, the iron electrode delivered only 0.02 Ah/g at the 1C rate, which was ten-times lower than that of a freshly formed electrode. Upon addition of sodium sulfide to the electrolyte, the discharge capacity of the iron electrode recovered completely (Figure 1). In addition, by limiting the input capacity to 0.25 Ah/g during charge, no further decay in capacity was observed (Figure 1). The discharge rate capability of the electrode also improved significantly after the addition of sodium sulfide to the electrolyte (Figure 2). The cycling behavior of the iron electrode was also observed to be dependent upon other factors such as the technique of electrode preparation, and the presence of electrode and electrolyte additives. An understanding of the capacity fade of the iron electrode and approaches to overcome the loss of discharge capacity during prolonged cycling will be presented. Acknowledgement The research reported here was supported by the U.S. Department of Energy ARPA-E (GRIDS program, DE-AR0000136), the Loker Hydrocarbon Research Institute, and the University of Southern California. References: 1. S. R. Narayanan, G. K. S. Prakash, A. Manohar, B. Yang, S. Malkhandi, A. Kindler, Solid State Ionics, 216, 105 (2012). 2. Z. Yang, J. Zhang, M. C. W. Kintner-Meyer, X. Lu, D. Choi, J. P. Lemmon, Chem. Rev., 111, 3577, (2011). 3. A. K. Manohar, C. Yang, S. Malkhandi, G. K. Surya Prakash, S. R. Narayanan, J. Electrochem. Soc. 160, A2078 (2013). S. U. Falk, A. J. Salkind, Alkaline Storage Batteries, John Wiley (1969).